1. Field of the Invention
The present invention relates to an electrochemical separation membrane and the manufacturing method thereof, especially to the manufacturing method using ceramics precursors to produce ceramic particles by hydrolysis for forming a ceramic-polymer composite membrane having the inorganic particles distributed uniformly in polymer material, and having low internal resistance and high stability in circle charge/discharge.
2. The Prior Arts
The traditional secondary batteries, such as the nickel-cadmium batteries and the nickel-hydride batteries, have been replaced by a secondary lithium ion battery since the latter was presented to the public due to its advantages of high energy density and long lifespan. With the technology of the battery design and materials in existence and the development of new materials, the application fields of the secondary lithium ion battery are broadened.
In the market of consumer electronics and electric vehicles, the most important evaluating item is safety; therefore, raising the safety of the safety-related materials such as the separation membrane is the key work in battery design. Therefore, the quality indexes such as strength, thickness, distribution of micro holes and thermal actuating of the separation membrane determine the factors such as the capacitance of battery, life circle and lifespan of battery and safety. Therefore, the development of the separation membrane becomes more important while the marketing is developing.
Recently, most of the secondary lithium ion batteries use porous polyolefin polymers as the separation membrane, wherein the polyolefin polymers include polypropylene (PP), polyethylene (PE) and PP/PE/PP layer-laminated. The polyolefin polymers have advantages of low-cost, good mechanical strength and good chemical stability. Manufacturing methods of the separation membrane are divided into dry and wet type methods. The processes of the dry type method are disclosed in the content of U.S. Pat. Nos. 5,952,120, 6,207,053 and 6,368,742. Those methods use polyolefin polymers, i.e. PE, PP and PP/PE/PP layer-laminated, as the main materials for manufacturing the separation membrane. For the lithium ion battery system, since the polarity of the polyolefin polymers is very low, such that the wetting effect of electrolytes to the separation membrane is not good, and the ion electric conductivity of whole lithium ion battery is lower than the ion electric conductivity of the electrolytes. In order to improve the affinity between the polyolefin polymers and electrolyte and improve the wetting effect. In some researches, the surface properties of the polyolefin polymers are modified to increase wetting effect, for example, the U.S. Pat. No. 6,322,923 disclosed covering a gelatinous polymer on the polyolefin porous membrane to enhance the wetting effect. Chinese patent publication No. 101826606A disclosed a polytetrafluoroethene lithium ion battery separation membrane and its manufacturing method, which uses a polytetrafluoroethene porous membrane as a base material, then a polymer is formed on one or two surfaces of the base material by dipping, coating or sprays coating, and finally a composite membrane is obtained after drying and thermal-pressing shaping. The battery separation membrane has the long service lifespan and safety based on its chemical stability, thermal stability and antioxidative activity.
The manufacturing method of the polyolefin separation membrane is that firstly extruding the melting materials into thin film firstly, and then stretching the thin film in a single direction or bi-direction. In the stretching process, the lamella structure of hard elastic materials, which is arranged in parallel and perpendicular to the extruding direction, are stretched to form micro holes. Finally, those micro holes are fixed by a thermal boarding process. The cost of the dry type method is low, but the ultimate price of the separation membrane is still high because the conditions of processes are strict to meet the specification of the secondary lithium ion battery. The shapes of micro holes are straight, so that short circuit may occur in the cathode end of the secondary lithium ion battery because the separation membrane is pierced by the lithium crystal. In order to solve the safety problem, manufacturing a thermal resistance layer or adding inorganic particles are generally used. For example, U.S. Pat. No. 7,087,343 describes that a polyolefin porous membrane having a low melting point is combined with a non-woven fabrics base material with a high melting point by using a swelling polymer as adhesive, wherein the non-woven fabrics base material can be further coated with a nano-oxides serving as a heat-resistance layer.
As described above, the manufacturers of the separation membrane understand the disadvantages in thermal stability of traditional separation membrane (the main material is PE) recently, and try to improve the safety in some ways, among which the technology of inorganic composite separation membrane is mostly focused. US patent publication No. 2012/0,094,184 A1 disclosed that a separation membrane with good thermal stability is formed by coating a mixture solution including polymers, dispersant and ceramic particles with different sizes on a non-woven fabric base material. US patent publication No. 2012/0,090,758 A1 disclosed smashing and dispersing the ceramic particles by ball-milling. Those prior arts described above disclosed that the polymer solution and ceramics particles not dissolved are forced to mix uniformly by using the dispersant or ball-milling. However, the mixing effect is not noticeably obvious and the particle size of the ceramic particles is hard to control in the processes.
U.S. Pat. No. 7,959,011 disclosed that a composite layer is made of a PET non-woven fabric mixed with ceramic particles. The ceramic particles are added directly or formed after hydrolyzing the metal alkoxide. Since an inorganic layer is formed between the metal oxide and PET after continuous dipping, drying and sintering, such that the separation membrane has higher thermal stability and deform resistance under heating, and does not shrink and melt under 200° C. Therefore, the safety of the power batteries is improved. However, the cohesive strength between the composite layer and base material membrane is not good enough, and the stability of separation membrane is also not good.
Chinese patent publication No. 101481855A disclosed a manufacturing method of SiO2/polyvinylidene nano-composite fabric membrane, in which sol-gel principle is applied to change the properties of the nano SiO2 particles, co-mix with the nano SiO2 particle and the polyvinylidene, and finally manufacture the nano-composite fabric membrane with the electrospinning technology.
US patent publication No. 2010/0,316,903 A1 also describes manufacturing method of separation membrane, in which slurry including adhesive and ceramic particles is coated on a surface of a porous base material firstly, wherein the adhesive is a cross-linking polymer, such that the adhesive and the porous base material are cross-linked after slurry coating and the adhesion strength between the adhesive and the base material is enhanced. US patent publication No. 2012/0,015,254 A1 enhances the adhesion strength by other method, in which slurry including adhesive and ceramic particles with dielectric coefficient higher than 5 is coated on a porous base material firstly, and then a polymer solution is coated on the outside by the electrochemical method to form a second coating layer for covering the base material and enhancing the stability of the separation membrane.
Most of the prior arts coat slurry having ceramic particles on the two sides of the separation membrane to form protective layers. The technology has problems of low adhesive strength and non-uniform distribution of ceramic particles, and the performance of the separation membrane is affected. Some researches use polymer second cross-linking or covering to improve the adhesive strength by electrochemical coating. However, those methods make the process difficult and complicated. Some researches prepare the ceramic particles by sol-gel method, and the present invention prepares the polymer solution distributing uniformly by using the sol-gel characteristics of the metal alkoxide which serve as the precursors, such that the ceramic particles are produced by hydrolysis after coating to form the membrane, and the final product is a ceramic-polymer composite membrane with micro-holes, which has enough adhesive strength and the ceramic particles therein are distributed uniformly.
As described, most of prior arts add the ceramic particles directly which are coated on two sides of the membrane to form protective layers, and the accuracy of membrane thickness and the distribution of ceramic particles are hard to control. Therefore, an objective of the present invention to provide a manufacturing method of a ceramic-polymer composite membrane with simple processes so as to improve the disadvantages encountered in the prior arts.